The strange haze of the Milky Way galaxy

Trent Nouveau, 14th February 2012

New images from the European Space Agency’s Planck mission have revealed previously undiscovered islands of star formation, along with a mysterious haze of microwave emissions in our Milky Way galaxy.

"The images reveal two exciting aspects of the galaxy in which we live," explained Planck scientist Krzysztof M. Gorski from NASA's Jet Propulsion Laboratory in Pasadena, Calif., and Warsaw University Observatory in Poland.

"They show a haze around the center of the galaxy, and cold gas where we never saw it before."

To be sure, the new images depict the entire sky, which is dominated by the murky band of our Milky Way galaxy.

One of them shows the unexplained haze of microwave light - which was previously hinted at in measurements by NASA's Wilkinson Microwave Anisotropy Probe (WMAP).

"The haze [originates] from the region surrounding the center of our galaxy and looks like a form of light energy produced when electrons accelerate through magnetic fields," said Davide Pietrobon, another JPL Planck scientist.

"We're puzzled though, because this haze is brighter at shorter wavelengths than similar light emitted elsewhere in the galaxy," Gorski noted.

According to Greg Dobler, a Planck collaborator from the University of California in Santa Barbara, Calif, several explanations have been proposed for this unusual behavior, including higher numbers of supernovae, galactic winds and even the annihilation of dark-matter particles.

The second all-sky image is the first map to show carbon monoxide over the whole sky.

Cold clouds with forming stars are predominantly made of hydrogen molecules, difficult to detect because they do not readily emit radiation. Carbon monoxide forms under similar conditions, and though it is rarer, the gas emits more light. Astronomers can use carbon monoxide to identify the clouds of hydrogen where stars are born.

Surveys of carbon monoxide undertaken with traditional radio telescopes on the ground are quite time-consuming, and so are limited to portions of the sky where clouds of molecules are already known or expected to exist. In contrast, Planck scans the entire sky, allowing astronomers to detect the gas where they weren't expecting to find it.

Planck's primary goal remains observing the Cosmic Microwave Background, the relic radiation from the Big Bang, and to extract its encoded information about what our universe is made of, and the origin of its structure.

However, this relic radiation can only be reached once all sources of foreground emission, such as the galactic haze and the carbon monoxide signals, have been identified and removed.

"The lengthy and delicate task of foreground removal provides us with prime datasets that are shedding new light on hot topics in galactic and extragalactic astronomy alike," added Jan Tauber, Planck project scientist at the ESA.